A mass dragnet for a teen bombing suspect emptied the streets of Boston on Friday, after a frenzied night of violence left the fugitive's accomplice brother and a police officer dead.
Thousands of police went "door-to-door, street-to-street" in the suburb of Watertown hunting for 19-year-old Dzhokhar Tsarnaev, who is alleged to have carried out Monday's deadly Boston Marathon attack with his brother Tamerlan.
The brothers, who are of Chechen origin, are said to have killed a police officer in one shootout before Tamerlan, 26, was hit in a second gun battle in which a second officer was seriously wounded.
Tsarnaev escaped the night-time posse and eluded capture so far Friday when authorities ordered people across the Boston area to stay behind locked doors. There were widespread fears that the teen had explosives strapped to his body.
Public transport in the city and Amtrak trains between Boston and New York were halted, while schools and universities, such as Harvard and Massachusetts Institute of Technology, were closed along with businesses. Barely a bar or restaurant was open in the ghostly city center streets.
State police chief Timothy Alben said "we are progressing through this neighborhood, going door-to-door, street-to-street," as police scoured the town of Watertown in search of the suspect.
Tsarnaev's family and friends appealed for him to surrender to police, who chased him armed with shotguns and automatic rifles, with the backing of helicopters and armored personnel carriers.
The brothers are ethnic Chechen Muslims who moved to the United States about a decade ago. Their social media pages appeared to express sympathy with the struggle of Chechnya, which has been ravaged by two wars since 1994 between Russia and increasingly Islamist-leaning separatist rebels.
A citywide alert issued after two bombs tore through crowds at the Boston Marathon, killing three people and wounding about 180, reached a fever pitch in the hours after the Federal Bureau of Investigation (FBI) released pictures of the two suspects on Thursday.
The brothers tried to rob a convenience store near MIT in Cambridge late Thursday. A police officer was killed in a campus shootout minutes later, according to authorities.
The suspects then carjacked a Mercedes, setting off a police chase to Watertown. The two hurled explosives out of the window before the elder brother was shot. He died of bullet wounds and injuries from explosives strapped to his body, a doctor said.
A police officer was wounded in the clash and Tamerlan's use of explosives sparked the fears that his brother also had bombs on his body.
Boston Police Commissioner Ed Davis described the younger Tsarnaev -- who was an all-star wrestler in high school -- as "armed and dangerous."
"We believe this to be a terrorist, we believe this to be a man who has come here to kill people."
Massachusetts governor Deval Patrick extended an order to the public to "shelter in place," saying it was important for people to "keep the doors locked" because of the danger posed by the fugitive.
The suspects' father Anzor Tsarnaev told Russia's Interfax news agency his sons had been "set up by the secret services because they are practicing Muslims."
An uncle, Ruslan Tsarni, said his nephews "put a shame on the entire Chechen ethnicity" and that the teenaged Dzhokhar should give himself up and seek "forgiveness".
The attack on the marathon, which sent a hail of nails and shrapnel into a crowd of thousands at the finish line, was the worst terror assault on the United States since the September 11, 2001 suicide airliner attacks.
Just hours before the manhunt, the FBI released pictures and video of the two suspects, appealing for help to identify them.
One -- now identified as Dzhokhar -- was wearing a white baseball cap and the other -- Tamerlan -- a black cap.
President Barack Obama, who was briefed Friday by his national security team on developments in Boston, has pledged that the "evil" bombers would be brought to justice.
"Yes, we will find you, and yes, you will face justice," Obama said Thursday at a service at Boston's Cathedral of the Holy Cross.
Boston has held emotional tributes to the dead -- eight-year-old Martin Richard, Boston University graduate student Lu Lingzi of China and Krystle Campbell, a restaurant manager. Obama paid tribute to all three at the service.
More than 100 of the wounded have left Boston hospitals and fewer than 10 of those still in hospital remain in critical condition. Some have horrific injuries and will require new operations, doctors said.